1. Field of the Invention
This invention relates to the field of instrumentation amplifiers, and particularly to error reduction techniques for use with current-mode instrumentation amplifiers.
2. Description of the Related Art
An instrumentation amplifier (IA) is a type of amplifier that has been specifically designed to have characteristics suitable for use in measurement and test equipment, such as low DC offset, high common mode rejection (CMR), and high input impedance. Such an amplifier is typically used when a user needs to look at a differential signal in the presence of a common mode signal, which can often be larger than the differential signal of interest.
One possible IA topology, described in co-pending patent application Ser. No. 11/154,135 assigned to the present assignee, is shown in FIG. 1. This current-mode topology includes differential input terminals VINP and VINN which receive a differential voltage VINP−VINN and are coupled to the input nodes 20, 22 of first and second buffer amplifiers 24, 26, respectively. Buffer amps 24 and 26 produce respective output voltages V1 and V2 at respective output nodes 28, 30, which vary with VINP and VINN. Buffer amp 24 includes a transistor MN1 biased with a bias current Ibias1=I3−I1 (provided by current sources 32 and 34, respectively) and connected between V1 and a circuit common point 35, and buffer amp 26 includes a transistor MN2 biased with a bias current Ibias2=I4−I2 (provided by current sources 36 and 38, respectively) and connected between V2 and the circuit common point.
A resistor 39 having a resistance R1 is connected between nodes 28 and 30 such that it conducts a current IR1 that varies with VINP−VINN. When so arranged, MN1 conducts a current Ia=Ibias1−IR1 when VINP>VINN and a current Ia=Ibias1+IR1 when VINP<VINN, and MN2 conducts a current Ib=Ibias2−IR1 when VINN>VINP and a current Ib=Ibias2+IR1 when VINN<VINP. Circuitry (here, MN3 and MN4) is provided which couples current Ia to the current input terminal 40 of a current mirror 42, and couples current Ib to the mirror's current output terminal 46—which also serves as the IA's current output node. A buffer amplifier 55 coupled at its inverting input to current output node 46 provides the IA's output voltage Vout. The non-inverting input of buffer amplifier 55 would typically be connected to a voltage source 56 which provides a reference voltage Vref, and a feedback path between its output and inverting input preferably includes components such as R2 and C2, to control the amplifier's gain and/or stability.
To minimize DC mismatch errors, the IA shown in FIG. 1 is preferably chopper-stabilized, with typically both the buffer amplifiers (using switches 60a and 60b) and signal current paths (using switches 64a and 64b) chopped using a two-phase chopping cycle. However, though chopping gives the IA a low average offset voltage, it also induces an output voltage rippled caused by the chopped currents. The chopping of the buffer amplifiers also results in a gain error due to capacitive charging and discharging.